The invention relates generally to X-ray tubes, and more particularly to cathodes for use within X-ray tubes.
An X-ray tube is, generally, used for a system which sees through the inside of human bodies or other objects of interest, such as a medical or an industrial diagnosis system. The X-ray tube is formed with a cathode, an anode and/or target and a vacuum enclosure which houses the cathode and the anode/target therein. By applying a high voltage between the cathode and the anode target, electrons emitted from the cathode side impinge on the anode target and thereby X-rays emanate from the anode/target which are then directed at the object of interest to produce the X-ray image of the object.
Certain X-ray tube architectures require long electron beam paths in an electrical field free region. In these prior art X-ray tubes 10 shown in FIG. 1, the cathode 12 is formed with an emitter 13 having a flat emitting surface 14 in a vacuum enclosure 11. When the emitter 13 in the cathode 12 is energized, the flat surface 14 is able to direct the beams of electrons 16 from the cathode 12 in a specified direction through a drift tube 17 towards the anode or target 18, i.e., in a straight line from the flat surface 14 towards the target 18. When the beams 16 strike the target 18, the target 18 emits X-rays in a specified direction toward the object to be imaged.
In situations where it is desired to increase the resolution and/or reduce the size of the location onto which the X-rays are to be directed, it is necessary to focus the beams 16 from the emitted from the flat surface 14 of the emitter 13 in the cathode 12 more closely onto the target 18 to the focal spot corresponding to the desired area of the object, as these beams 16 are directed perpendicularly from the surface 14 of the emitter 13. This is especially true in situations where the architecture of the X-ray tube 10 requires a long electron beam path between the cathode 12 and the target 18. To do so, prior art X-ray tubes 10 use a number of different structures and methods, including electromagnetic and electrostatic focusing, among other conventional methods. In one conventional prior art structure and method, the X-ray tube 10 includes a number of focusing elements 20 located between the emitter 13 and the anode or target 18. In operation, the focusing elements 20, such as quadrupole magnets, for example, are operated to effect the change the strength of an electric field in the drift tube 17. The resulting changes in the electric field strength alters the path of the electron beams 16 as they pass through the drift tube 17, enabling the beams 16 to be focused on a more narrow area or focal point 22 on the target 18.
However, the use of the focusing elements 20 to enable focusing of the electron beams 16 from the flat emitting surface 14 of the cathode 12 adds significant complexity and cost to the tube 10. Further, as shown in FIG. 2, these prior art X-ray tubes 10 employing flat emitters 13 in the cathodes 12 have inherent aberrations in the beams 16 emitted therefrom and cannot be focused with a resolution of less than 1.0 mm. Therefore, it is desirable to provide an improved X-ray tube structure and method of manufacturing the tube structure and the emitter to enable focusing of the electron beam without the need for additional electrical and/or magnetic focusing elements.